1. Field of the Invention
The present invention is directed to an apparatus for regulating the flow-through amount of a flowing medium, such as a ventilator for regulating the flow-through amount of a gas.
2. Description of the Prior Art
Ventilator arrangements are known in the art such as disclosed, for example, in U.S. Pat. No. 3,741,208. Ventilators of this type control the gas flow to or from a patient using an analog electronic circuit with negative feedback. An adjustable reference value for the chronological curve of the desired gas flow is generated by a rated value generator. A gas flow meter defines the actual value for the current gas flow, and converts this value into a corresponding electrical signal which, after linearization, is supplied to the input of a difference-forming stage. The reference value is supplied to the other input of the difference-forming stage by the rated value generator. The output signal of the difference-forming stage forms the proportional part of the regulating variable, which serves the purpose of driving a stepping motor. The stepping motor moves a closure element disposed in a gas conduit, by which the cross section of the internal opening of the conduit can be varied, so that the difference between the current gas flow and the desired gas flow is maintained as close to zero as possible. An integral component combined with the proportional component of the regulating variable in a known manner.
The ventilator disclosed in U.S. Pat. No. 3,741,208 was the first to successfully achieve various time-dependent respiration patterns in a reproducible manner.
Another ventilator is described in the brochure (Form AA-213, May, 1983) for the PB 7200 Microprocessor Ventilator of the Puritan-Bennett Corporation. In this ventilator, a digital regulation takes place wherein the current gas flow is measured in analog fashion using a hot-film manometer, and the analog value is digitized and intermediately stored for subsequent use in a microprocessor for calculating a regulating variable together with a value from a table for the desired gas flow, which is also stored and is used as the rated value. The regulating variable thus consists of a proportional component, a differential component, and an integral component to which a constant is added for the purpose of preventing seizing of the closure means at the valve opening. A solenoid value as disclosed, for example, in U.S. Pat. No. 4,463,332 is used as the valve. An oblong magnetic element is provided as the closure element, which is pressed against the valve opening by means of a resilient membrane, which exerts a linear spring force on the magnetic element. When a current is applied to the coil of the solenoid valve, a force opposite the spring force is generated as a function of the magnetic flux, which lifts the magnetic element from the valve opening to adjust the opening in accordance with the equilibrium position which is achieved. The structure of the valve is selected so that movement of the magnetic element is linearly proportional to the applied electrical current.
A single control circuit having negative feedback is provided in each of the above known ventilators for controlling the position of the closure element or closure means in order to regulate the gas flow.
An article entitled "Some Aspects of The Dynamic Behavior of Air Crew Breathing Equipment" appearing in Aerospace Medicine, Vol. 36, No. 11, November, 1965 describes a respiration simulator wherein the desired gas flow is not controlled via a value, but by means of a piston motion, with some degree of regulation being indicated. The gas flow, the position of the piston, and the speed of the motor with which the piston is driven are defined as the actual quantities. All three actual values, are compared to a single rated value, which is a reference signal generated by a so-called "wave-form generator" for the chronological curve of the desired gas flow. This takes place in a single control circuit, i.e., the electrical signals received corresponding to the various actual values are substracted in common from the reference signal. A faultless regulation is thus not possible under normal conditions, because the different actual values disturb each other. For example, the actual value for the gas flow may be above the rated value and have the effect of reducing the current flow, whereas the actual value for the position of the piston, by contrast, may indicate an excessively low gas flow and act in the opposite direction, i.e. toward an increase in the gas flow.